As a conventional technology of this kind, there is one disclosed in Patent Document 1. This conventional technology is applied to a hydraulic working machine, for example, a hydraulic excavator, and includes a first hydraulic pump and second hydraulic pump for delivering pressure oil to drive a working element which is movable up and down, for example, a boom, and a double-acting hydraulic cylinder operable by pressure oil, which has been delivered from these first hydraulic pump and second hydraulic pump, to drive the boom, specifically a boom cylinder. Also included are a first directional control valve for the working machine to control a flow of pressure oil to be fed from the first hydraulic pump to a bottom chamber or rod chamber of the boom cylinder, specifically a first directional control valve for the boom, a second directional control valve for the working machine to control a flow of pressure oil to be fed from the second hydraulic pump to the bottom chamber or rod chamber of the boom cylinder, specifically a second directional control valve for the boom, a control device for switchingly controlling the first directional control valve for the boom and the second directional control valve for the boom, and a jack-up selector valve to be switched when a pressure in the bottom chamber of the boom cylinder has reached a predetermined pressure.
The conventional technology further includes a flow rate control valve and a center bypass selector valve both of which, when the pressure in the bottom chamber of the boom cylinder has become the predetermined pressure or lower, are actuated in association with switching of the jack-up selector valve. As a result, the flow rate control valve is switched to permit feeding of pressure oil from the first hydraulic pump to the first directional control valve for the boom, while the center bypass selector valve closes a center bypass passage on a downstream side of the first directional control valve for the boom.
According to this conventional technology, when the boom is operated in a lowering direction and the bottom pressure of the boom cylinder has become lower than a switching pressure for the jack-up selector valve, in other words, the predetermined pressure, the flow rate control valve is switched to permit feeding of pressure oil from the first hydraulic pump to the first directional control valve for the boom, and in addition, the jack-up selector valve is also switched to close the center bypass passage. Owing to the cooperation of these flow rate control valve and center bypass selector valve, the pressure oil delivered from the first hydraulic pump is fed to the rod chamber of the boom cylinder via the flow rate control valve and the first directional control valve for the boom, thereby making it possible to allow the boom to produce strong pushing force such as jack-up force for a body.